1. Field of the Invention
The present invention relates to an improved process for manufacturing L-(-)-carnitine from starting compounds containing an asymmetrical carbon atom having a configuration opposite to that of L-(-)-carnitine. The process of the present invention overcomes the drawbacks of conventional processes which first convert a starting compound into an achiral intermediate, generally crotonobetaine or gamma-butyrobetaine, and then convert the achiral intermediate to L-(-)-carnitine. The process of the present invention uses D-(+)-carnitinamide or D-(+)-carnitinenitrile as preferred starting compounds.
2. Discussion of the Background
Carnitine contains a single center of asymmetry and therefore exists as two enantiomers, designated D-(+)-carnitine and L-(-)-carnitine. Of these, only L-(-)-carnitine is found in living organisms, where it functions as a vehicle for transporting fatty acids across mitochondrial membranes. Whilst L-(-)-carnitine is the physiologically-active enantiomer, racemic D,L-carnitine has conventionally been used as a therapeutic agent. It is now recognized, however, that D-(+)-carnitine is a competitive inhibitor of carnitine acyltransferases, and that it diminishes the level of L-(-)-carnitine in myocardium and skeletal muscle.
It is therefore essential that only L-(-)-carnitine be administered to patients undergoing haemodialysis treatment or treatment for cardiac or lipid metabolism disorders. The same requirement applies to the therapeutic utilization of acyl derivatives of carnitine for treating disorders of the cerebral metabolism, peripheral neuropathies, peripheral vascular diseases and the like. These disorders are typically treated with acetyl L-(-)-carnitine and propionyl L-(-)-carnitine, which are obtained by acylating L-(-)-carnitine.
Various chemical procedures have been proposed for the industrial-scale production of carnitine. Unfortunately, these procedures are not stereospecific and produce racemic mixtures of D-(-)- and L-(-)-isomers. It is thus necessary to apply resolution methods in order to separate the enantiomeric constituents of the racemate.
Typically, the D,L-racemic mixture is reacted with an optically active acid (e.g. D-(-)-tartaric acid, D-(+)-camphorsulfonic acid, (+)-dibenzoyl-D-(-)-tartaric acid, N-acetyl-L-(+)-glutamic acid and D-(-)-camphoric acid) to obtain two diastereoisomers which can be separated from each other. In the classic process disclosed in U.S. Pat. No. 4,254,053, D-(-)-camphoric acid is used as the resolution agent of a racemic mixture of D,L-carnitinamide, obtaining D-(+)-carnitinamide as a by-product, and L-(-)-carnitinamide which, by hydrolysis, gives L-(-)-carnitine.
However, these resolution procedures are complex and costly, and in all cases result in the production of equimolar quantities of L-(-)-carnitine and D-(+)-carnitine or a precursor thereof as by-product, having configuration opposite to that of L-(-)-carnitine. Several microbiological processes have recently been proposed for producing L-(-)-carnitine via stereospecific transformation of achiral derivatives obtained from the huge amounts of D-(+)-carnitine (or of a precursor thereof, such as D-(+)-carnitinamide) which are generated as by-products in the industrial production of L-(-)-carnitine.
These processes are generally predicated upon the stereospecific hydration of crotonobetaine to L-(-)-carnitine, and differ principally by virtue of the particular microorganism employed to accomplish the biotransformation of interest. See, for example, the processes disclosed in: EP 0 12 1444 (HAMARI), EP 0 122 794 (AJINOMOTO), EP 0 148 132 (SIGMA-TAU), JP 275689/87 (BIORU), JP 61067494 (SEITETSU), JP 61234794 (SEITETSU), JP 61234788 (SEITETSU), JP 61271996 (SEITETSU), JP 61271995 (SEITETSU), EP 0 410 430 (LONZA), EP 0 195 914 (LONZA), EP 0 158 194 (LONZA), and EP 0 457 735 (SIGMA-TAU).
On the other hand, JP 62044189 (SEITETSU) discloses a process for stereoselectively producing L-(-)-carnitine starting from gamma-butyrobetaine, which is in turn obtained enzymically from crotonbetaine.
All of these processes have several drawbacks. First, D-(+)-carnitine must first be converted to an achiral compound (crotonobetaine, gamma-butyrobetaine) before it can be used as the-starting compound in all of the aforesaid microbiological processes.
In addition, the microbiological procedures proposed to date have not proven practicable for manufacturing L-(-)-carnitine on an industrial scale for one or more of the following reasons:
(i) the yield of L-(-)-carnitine is extremely low; PA1 (ii) the microorganisms must be cultivated in a costly nutritive medium; PA1 (iii) the microorganism can only tolerate low concentrations [up to 2-3% (w/v)] of crotonobetaine; PA1 (iv) side reactions occur, such as the reduction of crotonobetaine to gamma-butyrobetaine or the oxidation of L---carnitine to 3-dehydrocarnitine.
These side reactions reduce the final yield of
L-(-)-carnitine.
In order to overcome all of the aforesaid drawbacks of the known processes, in the Italian patent application RM 92 A 000 915 filed on Dec. 21, 1992 in the name of the same applicants as the present application, not available to the public inspection at the filing date of this application, a process has been disclosed which allows high yields of L-(-)-carnitine to be obtained starting from a by-product having configuration opposite to that of L-(-)-carnitine (such as D-(+)-carnitinamide) with no need to first convert the starting by-product into an achiral intermediate.